Japanese Patent Application Publication No. 2000-130240 (JP-A-2000-130240) discloses a technique for diagnosing the degree of degradation of engine lubricating oil based on the difference in the light absorption loss at each of at least two monochromatic light sources with different wavelengths.
In recent years, there is growing tendency to use biofuels containing fuel produced from biomass as alternative fuels for vehicle engines. Since biofuels contain components having an unsaturated bond, they are easily oxidized as compared to conventional fuels. Oxidative degradation of biofuels produces acids which can corrode metal parts of the fuel system and so on. Also, biofuels can polymerize through reaction with oxygen and so on. When polymerization occurs, the viscosity of fuel increases, which may adversely affect the proper execution of fuel injection control.
When a vehicle has not been driven for along period of time or when the daily travel distance is so short that refueling is not done very often, the biofuel is left in the fuel tank for a long period of time. Then, degradation may proceed and adverse effects as describe above can occur. It is, therefore, desired to detect degradation of biofuel automatically in order to reliably prevent the adverse effects as describe above in vehicles using biofuel. However, a technique for detecting degradation of biofuel with high accuracy has not been established yet.